Air conditioning systems are typically designed and engineered to obtain specific results by using conventional components which operate within certain predetermined parameters. Specifically, as one essential component, air conditioning systems will include a refrigerant, such as freon, which is repeatedly cycled through a fluid line. Not surprisingly, several processes are involved as the refrigerant is moved through the system.
For an overview of the operation of an air conditioner system, it is helpful to consider one cycle. As a start point for the cycle, consider the refrigerant to be in its gaseous state. During each cycle, the gaseous refrigerant is elevated from a relatively low pressure to a high pressure condition by a compressor. The refrigerant is then passed through a condenser coil where it is condensed at high pressure into a liquid or semi-liquid state. Next, the high pressure liquid refrigerant is passed through an expansion valve which reduces the pressure on the refrigerant. The now low pressure liquid refrigerant is then passed to an evaporator coil where it evaporates, at the low pressure, back into a gaseous state. This completes the cycle. The cycle is then repeated. It is, of course, to be appreciated that the refrigerant completely fills the fluid line and that, at all times, portions of the refrigerant are at various points in the process.
From the user's viewpoint, it is important to note that as the refrigerant evaporates, heat from its surroundings is transferred to the refrigerant. As intended for air conditioning systems, the surroundings from which the heat is transferred is the air that is to be cooled by the system.
Heretofore, whenever it has been desired or necessary to test an air conditioning system for a malfunction or an inefficiency, testing of the system has been primarily a matter of evaluating the condition of the refrigerant in the fluid line of the system. Such an evaluation has required a physical invasion of the fluid line to determine the volume of refrigerant in the system, as well as its pressure and temperature at various points in the fluid line. Obviously, an invasive evaluation of an air conditioning system can be time consuming and, in many instances, quite difficult to perform. Furthermore, it may be unnecessary.
The present invention recognizes that a physical invasion of the fluid line is not necessary for a complete and thorough analysis or evaluation of an air conditioning system. Instead, it is appreciated that an engineering evaluation of a system's component efficiencies can be made by making proper psychrometric analyses. For the present invention, such analyses rely on basic thermodynamic principles.
By definition, enthalpy (H,h) is a thermodynamic property of a working substance which is associated with the study of heat of reaction, heat capacity and flow processes. Mathematically, enthalpy is defined as h=u+pv where u is the internal energy, p the pressure and v the volume of a system. With this in mind, it is important to know that heat (Q,q) is energy that is in the process of transfer between a system and its surroundings. This energy transfer results due to temperature differences. In the context of the present invention, the relationship between enthalpy and heat can be simply stated. Namely, the heat absorbed (or rejected) in a quasistatic isobaric (i.e. constant pressure) process is equal to the difference between the enthalpies of the system in the end states of the process. For example, consider the evaporator coil of an air conditioning system. The heat (q) which is transferred from the surrounding air to the evaporator coil, during a cooling of the air, is equal to the difference between the enthalpies of air at the evaporator inlet (h.sub.inlet) and at the evaporator outlet (h.sub.outlet). EQU q=h.sub.inlet -h.sub.outlet .DELTA.=h
A similar relationship holds for the condenser coil as well.
Due to the fact air conditioning systems are typically engineered so that the refrigerant used will transition between a fluid and a gaseous state, it is helpful to define two different types of heat pertinent to this transition. These are latent heat, which causes the change of state, and sensible heat, which does not. Specifically, latent heat is the heat which is required to change the state of a unit mass of a substance from a solid to a liquid, or from a liquid to a gas. Importantly, latent heat is not measured because it does not involve a change of temperature. Thus, without any change in temperature, the specific latent heat for a state transition is the difference in enthalpies of the substance in its two states. On the other hand, sensible heat is heat which effects a change in the temperature of a body and which is, therefore, detectable by the senses. With these definitions, it is now possible to further define the sensible heat ratio (SHR) as the ratio of latent heat to sensible heat in a process.
Using air tables well known to the skilled artisan, it is possible to determine the enthalpy of an air mass by taking readings of both the relative humidity and the dry bulb temperature of the air mass. For purposes of the present invention, the dry bulb temperature (T.sub.d) is taken to be the equilibrium temperature of the air-vapor mixture as indicated by an ordinary thermometer. Further, relative humidity (.phi.) is taken to be the ratio of the partial pressure of the water vapor in a mixture to the saturation pressure of the vapor at the same temperature. Relative humidity may also be defined as the ratio of the density of the vapor in the mixture to the density of saturated vapor at the same temperature.
As can be easily appreciated, any diagnosis of an air conditioning system will involve evaluating various operational data and comparing this data with standards established by the system manufacturer. Obtaining the proper data, however, can be painstaking and labor intensive.
In light of the above, it is an object of the present invention to provide an apparatus for diagnosing and monitoring a closed air refrigeration system which relies on enthalpy readings and which, therefore, can be used without invasively entering the refrigerant fluid line of the system. It is another object of the present invention to provide an apparatus for non-invasively diagnosing a closed air refrigeration system which can be used in either a mobile or a fixed base configuration for, respectively, making an instantaneous or a continuous evaluation of an air conditioning system. Still another object of the present invention is to provide an apparatus for non-invasively diagnosing a closed air refrigeration system which is easy to use, relatively simple to manufacture and comparatively cost effective.